Method to adjust photoelectric telescope to respond to constant object size



Feb. 9, 1965 Filed Jan. 10, 1962 P. M. KNAPP METHOD TO ADJUSTPHOTOELECTRIC TELESCOPE TO RESPOND TO CONSTANT OBJECT SIZE TRANSMISSION2 Sheets-Sheet 1 RF RF com: TRANSMITTER RECEIVER GENERATOR 1/ WEAPQI;COINCIDENCE HIT TRIGGER ATE GATE "mole/110R] I l I I8 5 i I I :EI; TR II m l I I ADJ AMPL "uETEcToR T T SOURCE I l I PHOTOELECTRIC TELESCOPE l--WEAPON TARGET r r 'r l p 0 I l6.

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PHILLIP M. KNAPP IIVVENTOR Feb. 9, 1965 P. M. KNAPP 3,169,191

METHOD To ADJUST PHOTOELECTRIC TELESCOPE T0 RESPOND TO CONSTANT OBJECTSIZE Filed Jan. 10. 1962 2 Sheets-Sheet 2 F 5 5/ l6. i v 5.9 7 i5 56 S54 68 64 F xi t 1% \l 1 W70 a? mi? THRESH. AMPL ADJ T0 MODULATOR FIG. 6O E 7 g 1 THRESHOLD E I I Q DISPLACEMENT PHILLIP M. KN INVE/V FIG. 8 W

ATmR/VEY United States Patent 3,169,191 METHOD TO ADJUST PHOTOIELECTRICTELE- gfzfifilll T0 RESPGND T0 QONS'IAN I OBJECT Phillip M. Knapp,Timonium, Md., assignor to Aircraft Armaments, Inc, (Jockeysville, Md, acorporation of Maryland Filed Jan. 10, 1962, Ser. No. 165,434 2 Claims.(Cl. 250-411) This invention relates generally to photoelectrictelescopes, and more particularly to the light responsive element insuch =telescopes.

Co-pending application Serial No. 73,777, filed December 5, 1960 andassigned to the same assignee as this application, discloses a HitIndicator system for use in training personnel in the use of weapons.Such system links a weapon with all targets by the use of an RF link,and the target at which the weapon is pointed by an IR link. The targetis provided with an IR source which is caused to flash by an RFinterrogation signal from the weapon. A photoelectric telescope at theweapon will see the flash only if the weapon is pointed at the target.Gne of the problems associated with this system arises from the factthat the object size of the telescope is a function of target range.That is to say, doubling the range increases the area of the field ofview by four times. This means that at close range (small object size)the weapon must be pointed directly at the target it the telescope is tosee the flash, while at increased range (large object size) the weaponmay be considerably misaligned with the target and still see the flash.

It is therefore an object of this invention to provide a photoelectrictelescope wherein the object size can be made substantially independentof target range over an interval of range so that the maximum allowableaiming error decreases with increasing target range. It is a furtherobject of this invention to provide a photoelectric telescope whereinthe object size is controlled by selective use of the output of thelight responsive element in the telescope.

Briefly, the photoelectric telescope includes the optical portion whichfocuses light on the light responsive element, an alloy junctionphotodiode is the last mentioned element, an amplifier for amplifyingthe output of the photo-diode, and a sensitivity adjustment for theamplifier. An alloy junction photodiode has a sensitive surface suchthat the amplitude of the output due to light being focused on a pointon such surface is a function of the displace ment of the point from thecenter of the sensitive area. By controlling the level of input to theamplifier which must be exceeded in order to obtain an out-put, controlis exercised over the displacement, from the center of the sensitivearea, of the point upon which light is focused that causes the diode toproduce an amplitude equal to the sensitivity of the amplifier. Thelocus of such displacement is, in effect, the image size of thetelescope, and through the optics, defines the object size. Thus,control of the sensitivity of the amplifier actually controls the objectsize.

If the response of the photodiode as a function of displacement isplotted at two values of range of a light source of constant intensity,it is possible to select a value of amplitude at which the ratio of theinterval over which each response exceeds the selected value ofamplitude (e.g., the ratio of the image sizes) is inversely proportionalto the ratio of the ranges. If the selected value of amplitude is madethe sensitivity level of the amplifier, the object size at both rangeswill be the same. In this manner, the object size in the interval ofranges selected remains substantially uniform.

Still other objects, features and attendant advantages will becomeapparent to those skilled in'the art from a reading of the followingdetailed description of one preferred embodiment and mode of practice ofthe invention, taken in conjunction with the accompanying drawingswherein:

FIGURE 1 is a block diagram of an over-all Hit Indicator systemutilizing an RF link and an IR link.

FIGURE 2 is a signal-time diagram for the system shown in FIGURE 1.

FIGURE 3 is a schematic which indicates how the angle subtended by atarget of constant size decreases with range.

FIGURE 4 represents a target at a certain range showing the locus ofpoints which defines an area within which the point of aim must lie forthe photoelectric telescope to have an output.

FIGURE 5 shows a photoelectric telescope mounted on the barrel of aweapon.

FIGURE 6 is the amplitude response of the photodiode taken along thecenter line of the sensitive surface of the photodiode of FIGURE 7.

FIGURE 7 is a top view of the sensitive area of an alloy junctionphotodiode.

FIGURE 8 is a sectional view taken along the center line of thephotodiode of FIGURE 7 and showing the leads attached thereto.

Reference is now made to FIGURE 1, which shows a hit indicator systemIt) to which the photoelectric telescope of this invention is adaptable.In operation, the Weapon trigger I1 is pulled when the firer believes hehas the target properly aligned with the weapon. This actuates modulator12 which turns on RF transmitter 13 cansing RF pulse A to betransmit-ted omnidirectionally from antenna 14. Pulse A is received atantenna 15 of each target and is detected in receiver 16 as pulse acoincident in time with pulse A. Pulse a triggers code generator 17which causes IR source 13 to flash at time t a predetermined timesubsequent to time t Each target has its own time interval, t -t If theweapon is pointed at a target, IR pulse B is detected by IR detector 19at the weapon as pulse 1) and amplified in amplifier 20. Pulse [2 causesmodulator 12 to pulse transmitter 13 producing at t an RF pulse C whichis radiated omnidirectionally from antenna 14. This pulse is detected ateach target 'by receiver 16 as pulse 0 coincident in time with the flashdesignated B from the target at which the weapon is pointed. Thus, thepulse c from the receiver is coincident with the pulse out of generator17 only it the weapon is pointed at the target. The outputs of thereceiver and generator are compared at 21, and if in coincidence, hitindicator 22 is actuated. The time interval t i is controlled -by thesetting of code generator 17 so that a number of targets may beaccommodated, each target having a different time interval associatedtherewith. Thus, while pulling the trigger at 11 causes the IR source ateach target to flash, only that target at which the weapon is pointedwill receive an RF pulse coincident with its IR flash. The abovedescription of the hit indicator system is by way of setting forth theenvironment within which the photoelectric telescope of the inventionmay operate. Since such system forms no part of this invention, nofurther description is believed to be necessary.

Reference is now made to FIGURE 3 which schematically illustrates thebasic difliculty with the photoelectric telescope. Point 39 representsthe eye of an observer looking at target 31 a distance R away, andtarget 32 a distance R away. Targets 31 and 32 are identical, and theangle subtended by target 31 is :1 and by target 32 is Angle 0: isgreater than angle 1x Assume now that each target has an IR source 33mounted thereon. It is obvious that it the field of view of the observeris 0:

movement of target 31 between the limits shown in broken lines in FIGURE3 is permitted without source 33 passing outside the field of view.However, at range R and the same angle 1x it is obvious that target 32may have a much larger movement before source 33 passes outside thefield of View. Only if the field of view were restricted to the angle :1would target movement at range R be the same as that at range R beforethe source passes out of the field of view.

Assume now that center line 34 is the optical axis of a photoelectrictelescope that is bore sighted with a weapon. It is apparent that withline 34 fixed, if target 31 were anywhere between the limits shown inFIGURE 3, the flash of source 33 would be detected. This is the same assaying that the weapon may be incorrectly aimed within plus or minus 11degrees of center line 34 and still detect a flash. The latter point isillustrated in FIGURE 4, which shows tank target 35 having light source33 atop the turret. Circle 36 represents the field of view (object size)of the telescope with center 37 being the optical axis of the telescope.Circle 36 just includes source 33. Circle 38 represents the envelope ofthe area swept out by circle 36 as the latter is rotated around thesource 33. In other words, the cross-hatched area of circle 38represents the vulnerable area (area within which a hit is indicated) oftarget 35. Unless circle 36 can be kept constant with range, thevulnerable area of target 35 increases at the range of target 35increases.

To maintain the object size substantially independent of range, thephotoelectric telescope of FIGURE 5 can be used. In FIGURE 5, the barrelof a weapon is indicated at 50, and the telescope at 51. The telescopeincludes tubular housing 52 rigidly connected to barrel by clamps 53.The forward end of housing 52 contains objective lens 54, and at thefocus of this lens is the sensitive surface 55 of photodiode assembly56. One end of tube 57 of assembly 56 is closed by glass seal 58, andthe other end by plug 59. Mounted inside tube 57 is alloy junctionphotodiode 60 which includes an n-type bar 61 of semiconductive materialwith a p-type alloy pellet 62 alloyed thereto on the side remote fromsurface 55. Alloy junction photodiodes are well known, and an example ofsuch a diode is shown and described on page 10-13 in Handbook ofSemiconductor Electronics, edited by L. P. Hunter, published byMcGraw-Hill Book Company, Inc., 1956.

Lead 63, in the form of a rigid rod, is connected to pellet 62 andextends through plug 59 and ends in terminal 64. Lead 63 serves torigidly support the diode. Lead 65 is in the form of a flat bar that isohmically connected to face 55 at 66 adjacent one edge of bar 61.Connection 66 is remote from the effective portion of sensitive area 55.Lead 65 passes through plug 59 and ends in terminal 67. Lead 63 connectsterminals 64, 67 to amplifier 69, which is compatible for use withphotodiode 60 Amplifier 69 has an adjustment 70 which controls thesensitivity level of the amplifier Thus, for a given setting ofadjustment 70, there is no output from the amplifier except when theamplitude of the output from photodiode 60 exceeds the sensitivity, andthe amplifier with its sensitivity adjustment thus serves also as aselectively variable threshold signal control means or gate The sectionof the photodiode shown in FIGURE 8 is greatly enlarged However, for areasonable sized objective lens, the circle of confusion on surface 55of base bar 61 can be made as small as 0.001" in diameter with theresult that the effective sensitive area (surface 55) can be made over3600 times larger than the area of the circle of confusion. Using apoint source, the image of the source can be made to traverse thesensitive surface, passing through the center 71 of the surface, atwhich point the response is a maximum. The response of the photodiode asa function of the displacement from the center of the sensitive area ofthe image of a point source is shown in FIGURE 6 for ranges R and R Thesource has a l constant intensity output and the difference in the peakamplitudes of the responses is due to the decrease in intensity ofradiation reaching the sensitive surface as the source changes range.

If adjustment "iii is set so that the sensitivity of the amplifier is ata volts, there will be no output from the amplifier unless the output ofthe photodiode exceeds a volts. As shown in FIGURE 7, locus 72 is anequipotential line of a volts for the response of a point source at arange R and locus 73 is an equipotential line of a volts for theresponse of a point source at a range R Thus, diameter S is really theimage size of an object at range R and S is the image size of an objectat range R It should be noted for a given sensitivity setting, the imagesize generally decreases with range. If the decrease in image size isproperly correlated with range, the object size can be made independentof range.

For a simple telescope, the image size is related to the optical size asfollows:

0.S. I.S. R O.S. I.S. R

If the object sizes are to remain constant, their ratio is unity. Hence,if S is the image size at range R and S is the image size at R Thevoltage a is selected to satisfy the last defined relationship. That is,the sensitivity of the amplifier is adjusted until the ratio of theintervals of displacement over which each response exceeds thesensitivity is inversely proportional to the ratio of the ranges atwhich the responses are obtained. As used herein, the term interval ofdisplacement for range R over which the response exceeds the sensitivitymeans 5;, the diameter of circle 72.

It should be understood that unless the response curves for thephotodiode for ranges in the interval (R R intersect voltage line a atjust the proper point, the object size will be slightly different atintermediate ranges. Experience shows that the object size increasesslightly at intermediate ranges. However, proper control of the contourof the alloy junction and the elevation of surface 55 may advantageouslybe used to control the profile of the response curves.

Where a target is at a fixed range, a simple adjustment of thesensitivity of the amplifier will control the image size of thetelescope. Through the optical relationships involved, this givescontrol over the object size. In this manner, the amount of angularpointing error of the telescope that achieves detection of a flash fromthe source can be controlled.

While other semiconductor diodes are suitable, the spectral response ofa germanium photodiode peaks in the infrared region (about 1.4 microns)making this type of diode particularly well suited for use as outlinedabove. To this end, infrared filter may be interposed between theobjective lens 54 and the photodiode. This serves to prevent saturationof the photodiode due to background light. The rapidity with which agermanium diode follows light of fluctuating intensity makes such adiode well suited for use Where the flash is of short duration.

While a simple telescope is shown here, it is obvious that the methoddisclosed herein could be used, in any optical system. Furthermore, thephotoelectric telescope per se has other uses than in the system shownin FIG- URE 1.

While the invention has been described with reference to one particularillustrative embodiment and mode of practice, it will be apparent tothose skilled in the art that various modifications and improvements maybe made without departing from the scope of the invention. Accordinglyit is to be understood that the invention is not to be limited by thespecific illustrative embodiment and described manner of practicespecifically set forth herein, but only by the scope of the appendedclaims.

What is claimed is:

l. A method for maintaining object size substantially independent ofrange in a photoelectric telescope that includes an alloy junctionphotodiode for producing an output when light is focused on thesensitive area thereof, an amplifier for amplifying the output of saidphotodiode, and a sensitivity adjustment for the amplifier forpreventing the latter from having an output except when the output ofsaid photodiode exceeds the value of the sensitivity comprising thesteps of: focusing light from a source of constant intensity on thesensitive area of the photodiode when located at two ranges andobtaining for each range the response of the photodiode as a function ofdisplacement on said sensitive area of the image of said source from thecenter of said sensitive area, and adjusting the sensitivity of theamplifier until the ratio of the intervals of said displacement overwhich each response exceeds the sensitivity of said amplifier isinversely proportional to the ratio of the two ranges.

2. A method for maintaining object size substantially independent ofrange in a photoelectric telescope that in- #3 cludes an alloy junctionphotodiode for producing an output when light is focused on thesensitive area thereof, signal control means connected to the output ofsaid photodiode, and a sensitivity adjustment for the signal controlmeans for preventing the latter from having an output except when theoutput of said photodiode exceeds the value of the sensitivitycomprising the steps of:

References Cited by the Examiner UNITED STATES PATENTS 2,560,606 7/51Shieve 250-211 2,740,901 4/5'6 Graham 25021l X 2,961,545 11/60 Astheimeret a1 250-203 2,965,762 12/60 Turck 250203 2,966,823 1/61 Trimble250-203 X 3,028,500 4/62 Wallmark 30788.5

RALPH G. NILSON, Primary Examiner.

WALTER STOLWEIN, Examiner.

1. A METHOD FOR MAINTAINING OBJECT SIZE SUBSTANTIALLY INDEPENDENT OFRANGE IN A PHOTOELECTRIC TELESCOPE THAT INCLUDES AN ALLOY JUNCTIONPHOTODIODE FOR PRODUCING AN OUTPUT WHEN LIGHT IS FOCUSED ON THESENSITIVE AREA THEREOF, AN AMPLIFIER FOR AMPLIFYING THE OUTPUT OF SAIDPHOTODIODE, AND A SENSITIVITY ADJUSTMENT FOR THE AMPLIFIER FORPREVENTING THE LATTER FROM HAVING AN OUTPUT EXCEPT WHEN THE OUTPUT OFSAID PHOTODIODE EXCEEDS THE VALUE OF THE SENSITIVITY COMPRISING THESTEPS OF: FOCUSING LIGHT FROM A SOURCE OF CONSTANT INTENSITY ON THESENSITIVE AREA OF THE PHOTODIODE WHEN LOCATED AT TWO RANGES ANDOBTAINING FOR EACH RANGE THE RESPONSE OF THE PHOTODIODE AS A FUNCTION OFDISPLACEMENT ON SAID SENSITVE AREA OF THE IMAGE OF SAID SOURCE FROM THECENTER OF SAID SENSITIVE AREA, AND ADJUSTING THE SENSITIVITY OF THEAMPLIFIER UNTIL THE RATIO OF THE INTERVALS OF SAID DISPLACEMENT OVERWHICH EACH RESPONSE EXCEEDS THE SENSITIVITY OF SAID AMPLIFIER ISINVERSELY PROPORTIONAL TO THE RATIO OF THE TWO RANGES.